(1) Field of the Invention
The invention relates to the general technical field of providing piloting assistance for aircraft flying at low altitude. In this kind of flight configuration, often close to obstacles and to the ground, it is necessary to have safety margins that are reliable when a pilot is following a path manually or with the help of an autopilot system. These margins, which are representative of the distance between the aircraft and the terrain, are displayed on a screen, e.g. in the form of a safety cordon, and they are absolutely essential, particularly when flying in low visibility.
The present invention relates more particularly to flying at low altitude and more precisely to following terrain at varying altitudes on a continuous basis, with an aircraft such as a rotorcraft, e.g. a helicopter, in order to avoid colliding with the terrain or with obstacles.
The following conventional abbreviations are used below:
Lidar (light detection and ranging);
Radar (radio detection and ranging);
AHRS (attitude and heading referential system);
GPS (global positioning system);
GNSS (global navigation satellite system), a term covering any satellite positioning system, including the global positioning system (GPS);
FOR (field of regard), measured in terms of the aperture angle of the acquisition window;
MSL (mean sea level);
WGS (world geodetic system);
HFoM (horizontal figure of merit), i.e. position error in the horizontal plane; and
VFoM (vertical figure of merit), i.e. position error in the vertical plane.
Furthermore, during a medical evacuation or when flying at low altitude under cloud cover, helicopters attempt to fly as close as possible to the terrain, while avoiding colliding therewith. In order to perform contour flying at low altitude, while avoiding collisions with the terrain, helicopter pilots fly under visual flight rules (VFR). Known means enable this type of mission to be performed, providing visibility is good, or else in poor visibility, but at altitudes that are not appropriate for all missions. Such altitudes are generally related to information in a terrain database in which potential obstacles are referenced.
(2) Description of Related Art
At present, two families of solutions are known for performing low altitude flying, while remaining as close as possible to the terrain and while avoiding obstacles.
One of the families relates to methods using terrain databases (elevation relative to a reference geoid (e.g. MSL or WGS84)), possibly together with databases of obstacles (geo-located together with their heights above the ground). Those methods are strongly dependent on geo-location means. For example, losing a GNSS system presents a major drawback for continuing a mission under initial conditions. Furthermore, there is the problem of lack of accuracy in “terrain” databases. Obstacles such as cables are not always accurately identified. In order to comply with flight safety margins, the helicopter is thus constrained to fly at an altitude that is too high relative to the relief.
Another family relates to methods making use of active telemeter sensors. Those methods present the drawback of not making it possible to anticipate turns that need to be performed and of not providing any predictive aspect concerning the path to be followed over the long term. One method making use of active telemeter sensors is described for example in Document FR 2 886 439. The method described nevertheless requires flight to be performed at high altitude when visibility is poor. That method also suffers from wave-reflection problems of the kind that are inherent to telemeter sensors.
In addition, such methods are very dependent on the quality of the telemeter sensor used. For example, such sensors have varying ranges (from 500 meters (m) to 2000 m), with cables being detected in Lidar mode, but not necessarily in Radar mode, and with other obstacles being detected regardless of the weather in Radar mode, but not in Lidar mode. Such methods thus increase the stress and the workload on the pilot.
Document U.S. Pat. No. 3,245,076 discloses an autopilot system enabling a safety curve to be determined at a distance from the aircraft, and more precisely with the help both of its speed vector, and of a distance corresponding to the minimum distance that must be maintained between the aircraft and detected relief. The system described also determines upper and lower curves located on either side of the safety curve. As a function of the appearance of obstacles that are referenced positively or negatively relative to the safety curve, between the lower and upper curves, the system calculates angles for pointing the nose down or up that are compatible with the maneuverability of the aircraft.
Document FR 2 712 251 describes a method of assisting the piloting of an aircraft for flying at low altitude, which method consists in detecting dangerous obstacles in relief. The method is based in particular on the maneuvering capability of the aircraft, on the basis of which a fictitious or potential curve is calculated that is tied to the aircraft and that is associated with an optimum theoretical path for overflying an obstacle in a vertical plane. That optimum theoretical overflight path is recalculated in each angular sector of the FOR while taking into consideration its highest obstacle, e.g. as detected by a telemeter sensor.
Document FR 1 374 954 describes associating a radar and a computer to operate continuously to determine the situation of an aerodyne relative to the ground and to issue nose-down or nose-up orders.
In addition to the Documents FR 2 886 439, U.S. Pat. No. 3,245,076, FR 2 712 251 (=EP 0 652 544), and FR 1 374 954, other documents may be mentioned.
Thus, Document U.S. Pat. No. 5,892,462 describes an adaptive type system for avoiding collision with terrain. Parameters are taken into account from various sources in order to consolidate a terrain-avoiding algorithm, those parameters including telemeter measurements or map-based data, but without seeking to construct a safety cordon over angular sectors.
Document US 2008/0243383 describes a terrain collision avoiding system that incorporates parameters from various sources.
Document U.S. Pat. No. 7,633,430 describes a terrain awareness warning system (TAWS) for aircraft, which system incorporates various parameters including on-board radar returns.
Document US 2003/195672 describes a flight management system including an augmented three-dimensional (3D) display of terrain.
Documents US 2006/0235581, FR 2 658 636, U.S. Pat. No. 6,317,690, and US 2008/243383 may be considered.
When a telemeter sensor is used, known methods are also highly dependent on the quality of the telemeter sensor and are unsuitable for mitigating any failure of said telemeter sensor. The pilot may thus be in a situation in which it is not possible to use a safety cordon, either because it is not available, or else because it is degraded by data that is wrong or inaccurate. The methods described also require flying to take place a high altitude when visibility is poor.
Problems of wave reflections that are inherent to telemeter sensors are not overcome by using said methods.
All of the measurements from geo-locating means are often not taken into account at present, in particular the measurement errors delivered by GNSS means such as HFoM or VFoM giving information about horizontal and vertical measurement errors are often not taken into account when calculating a path for low altitude flights.